Electrophotographic laser printing employs a toner containing pigment components and thermoplastic components for transferring a latent image formed on selected areas of the surface of an insulating, photoconducting material to an image receiver, such as plain paper, coated paper, transparent substrate (conducting or insulative), or an intermediate transfer medium.
There is a demand in the laser printer industry for multi-colored images. Responding to this demand, designers have turned to liquid toners, with pigment components and thermoplastic components dispersed in a liquid carrier medium, usually special hydrocarbon liquids. With liquid toners, it has been discovered that the basic printing color (yellow, magenta, cyan, and black) may be applied sequentially to a photoconductor surface, and from there to a sheet of paper or intermediate medium to produce a multi-colored image.
Specific morphologies of phthalocyanine (Pc) pigment powder have been known to exhibit excellent photoconductivity. These phthalocyanine pigments have been used as a mixture in polymeric binder matrices in electrophotographic photoconductors, deposited on a conductive substrate. In these phthalocyanine/binder photoconductors, the photogeneration of charge and the charge transport occur in the particles of the phthalocyanine pigment, while the binder is inert. Therefore, the photoconductor may be made of a single layer of phthalocyanine/binder. These single-layer photoconductors are known to be very good positive (+) charging OPCs due to the hole (positive charge) transportability of the phthalocyanine pigment.
In these single-layer photoconductors, then, there is no need to add charge transport molecules, nor to have a separate charge transport layer. The phthalocyanine pigment content may be in the range of about 10 to 30 wt %, high enough to perform both charge generation and charge transport functions, with the binder content comprising the balance, i.e., in the range of about 90 to 70 wt %. The single photoconductor layer is usually more than about 3 micrometers (.mu.m) thick in order to achieve the required charge acceptance and resulting image contrast.
It would be desirable to provide a phthalocyanine-type positive-charging OPC which exhibits stable electrical properties, including charge acceptance, dark decay and photodischarge, in a high cycle, high severity electrophotographic process, operating at elevated temperatures, on the order of about 35.degree. to 75.degree. C. Modern digital imaging systems wherein the writing head is an LED array or a laser diode have very high light intensities (about 2 to 3 mW/cm.sup.2) over very short exposure time spans (less than 50 nanoseconds), resulting in severe conditions for the OPC compound compared to optical input copiers with light intensities between about 10 to 30 erg/cm.sup.2 and exposure times between several hundred microseconds to milliseconds. These light sources operate in the range of about 700 to 1100 nm, which, due to the absorbance of the phthalocyanine compounds in the higher end of this range, is why these compounds are employed.
Unfortunately, there is no product on the market today which provides the stable electrical properties described above. This is because the phthalocyanine-type positive-charging OPC exhibits instability when it is frequently exposed to the corona charger and the intense light source in the electrophotographic process at elevated operating temperatures exceeding 35.degree. C. The instability is more pronounced at the strong absorption, high light intensity, short exposure time conditions required for the laser printing process. The instability is exhibited in the significant increase of the dark decay after a small number of repeat cycles of laser printing. Also, the instability is exhibited in the decrease in surface potential. These instabilities cause deleterious changes in image contrast, and raise the issue of the reliability of image quality.
These instabilities in the phthalocyanine/binder photoconductor appear to be independent of the chemical structure or morphology of the pigment. Instead, they appear to be dependent on the nature of the contact between individual pigment particles. These are recent observations, and there is no published report or suggestion in the prior art of these observations or how to effectively address and solve the problem of photoconductor instability in the high cycle, high severity electrophotographic process.
Phthalocyanine pigments having specific morphology associated with particle size in sub-micrometer range have been observed to show different effects, depending on the type of the binder, such as agglomeration or aggregation. These properties are associated with the unstable dispersion of the pigment in the binder due to the poor compatibility between the two components. The above-mentioned unstable dispersion can cause the problem of non-uniformity of the coating, resulting in defects of the xerographic image quality, such as high noise and poor resolution. The poor dispersion of these pigments in binder also causes the unstable performance of the device, such as reduced life at different operating environments (ambient and elevated temperatures). The specific morphology with sub-micrometer particle size can be found in the following types of phthalocyanine pigments: the metal-free crystalline forms (a-, .beta.-, .tau.-, and x-H.sub.2 -phthalocyanines), .alpha.-copper phthalocyanine, .alpha.-titanyl phthalocyanine, Y-titanyl phthalocyanine, amorphous titanyl phthalocyanine, .alpha.-tetrafluorotitanyl phthalocyanine, .alpha.-haloindium phthalocyanines (halo=Cl, Br, I, F), .alpha.-vanadyl phthalocyanine, .alpha.-zinc phthalocyanine, .beta.-zinc phthalocyanine, x-magnesium phthalocyanine, .alpha.-chloro-alumium phthalocyanine, and hydroxygallium phthalocyanine.
When conventional binders for the phthalocyanine pigment which do not contain a hydroxy group, such as acrylic resins, vinyl polymers, including polyvinylacetate, polystyrene, polyesters, polyamides, polyimides, polycarbonates, methylmethacrylates, polyurethanes, polyureas, melamine resins, polysulfones, polyarylates, diallylphthalate resins, polyethylenes, and halogenated polymers, including polyvinylchloride, polyfluorocarbon, etc., are used, acceptable charge acceptance and photodischarge are obtained. However, among these polymers which result in good performance for charge acceptance and photodischarge, none of them exhibit the desirable thermal stability under the LED array or laser diode exposure conditions. Also, any binders, and accompanying solvents, which do not form a stable dispersion with the phthalocyanine pigment usually exhibit very low charge acceptance, high residual voltage, or dark decay, and are therefore unacceptable.
The conventional polymeric binders, such as polycarbonates, polyesters, phenoxy resin, phenolic resin, polystyrene, polyvinyl toluene, polyvinyl carbazole, polyimide, and the like, contain unsaturated rings. On the other hand, some functional groups in the binder, especially hydroxy groups (--OH) and thiols (--SH), as well as &gt;NH, --NH.sub.2, &gt;N--, seem to exhibit strong interactions (e.g., hydrogen bonding) with the lone pair nitrogen of the phthalocyanine molecules. These interactions are observed to restrict the photoresponse of the photoconductor devices under space charge limited condition, such as exposing to strong light intensity in a very short time of several tens of nanoseconds.
Preferably, desirable electrophotographic performance may be defined as high charge acceptance of about 30 to 100 V/.mu.m, low dark decay of less than about 5 V/sec, and photodischarge of at least 70% of surface charge with the laser diode beam of 780 nm or 830 nm frequency, through the optical system including beam scanner and focus lenses, synchronized at 0.05 msec for each beam.
Thus, there remains a need to provide binders for the positive single layer OPC using sub-micrometer morphology phthalocyanine pigment as a photoconductive element to satisfy (a) stable dispersion, (b) high photoresponse to laser exposure, and (c) stable performance over a wide range of elevated operating temperatures (about 35.degree. to 75.degree. C.).